This Exploratory/Developmental Research (R21) application proposes to study the phenomenon of turbidity suppression optical phase conjugation (TS-OPC) and explore its uses for biomedical applications. TS-OPC - a biophotonic phenomenon recently observed by my group, is based on the use of optical phase conjugation to suppress tissue turbidity. Unlike absorption, elastic optical scattering is a time reversible process. As such, a time reversed copy of the scattered field should be able to retrace its trajectory through the scattering medium and recover the original light field. Our initial experiments revealed that this phenomenon is surprisingly robust - we were able to observe the phenomenon with a chicken tissue section of thickness 5mm and a wavelength of 532 nm. Our initial investigations also point to the potential utility of the method for several applications: optical powering of miniature sub-dermal implants, improving photodynamic therapy procedures, tissue absorption/scattering interrogation, etc. The focus of the proposed research is to attain a more complete understanding of the technique's working principle, streamline its implementation in reflection mode and begin initial investigations into its use for enhanced and targeted light delivery into tissues for photodynamic applications. The specific aims of the proposed research are: 1. Develop a quick, efficient and simple reflection mode TS-OPC system. 2. Experimentally determine the limits of the reflection mode TS-OPC method. 3. Develop a theoretical framework for understanding the TS-OPC method. 4. Begin initial investigations into the use of TS-OPC for enhanced and targeted light delivery into tissues - useful for photodynamics therapy (PDT). Narrative for Optical phase conjugation project PUBLIC HEALTH RELEVANCE: The phenomenon of turbidity suppression via optical phase conjugation can potentially be adapted for clinical diagnosis and therapeutics. The funding of the proposed research will allow us to better understand this newly discovered phenomenon and allow us to begin exploring one of its more promising applications - the enhancement of light delivery during photodynamic therapy procedures.